The present disclosure relates to a hydraulic control device of an automatic transmission including a plurality of engagement elements that is mounted, for example, on a vehicle, and more in detail, to a hydraulic control device of an automatic transmission that can engage and disengage engagement elements using double-chamber hydraulic servos each including a plurality of hydraulic oil chambers for one of the engagement elements.
Conventionally, in a stepped automatic transmission mounted, for example, on a vehicle, a hydraulic control device controls engagement states of a plurality of engagement elements (clutches and brakes) to establish a transmission path in a speed change mechanism at each shift speed, so that multi-speed transmission is achieved. In the stepped automatic transmission and the hydraulic control device described above, hydraulic servos are used to engage and disengage the engagement elements. Such a hydraulic servo commonly includes one hydraulic oil chamber for each of the engagement elements.
For example, in a multi-speed automatic transmission, due to structures of gear trains, engagement elements to be engaged at a first forward speed or a first reverse speed have larger torque shares than those of engagement elements to be engaged at shift speeds other than these shift speeds. For this reason, in the case where a hydraulic pressure is supplied from a linear solenoid valve to the hydraulic oil chamber of each of the engagement elements to be engaged at the first forward speed or the first reverse speed, the linear solenoid valve needs to have a higher gain (displacement of a hydraulic pressure output with respect to displacement of a current command value) than the case in which the hydraulic pressure is supplied to hydraulic oil chambers of other engagement elements. This may degrade controllability of the engagement elements to be engaged at the first forward speed or the first reverse speed. The engagement elements to be engaged at the first forward speed or the first reverse speed have larger torque shares, so that the supply pressure of an oil pump needs be higher, leading to a design in which a load on a pump is high. In the case of establishing the shift speeds other than the first forward speed and the first reverse speed that use such engagement elements, the supply pressure of the oil pump need not be so high because of smaller torque shares. Despite of this, the high load on the pump inhibits improvement in fuel consumption.
On the other hand, a double-chamber hydraulic servo has been developed which includes a plurality of hydraulic oil chambers for each engagement element (refer to Japanese Patent Application Publication No. 2007-64399). The double-chamber hydraulic servo includes first and second hydraulic oil chambers, which can be supplied with respective engagement pressures using hydraulic pressure supplying paths separate from each other. The engagement element to be engaged can have different torque capacities between a case of supplying an engagement pressure only to the first hydraulic oil chamber and a case of supplying the engagement pressures to both of the two hydraulic oil chambers. With this structure, both of the two hydraulic oil chambers are supplied with the engagement pressures during engagement when a large torque capacity is required, and only the first hydraulic oil chamber is supplied with the engagement pressure during engagement when the large torque capacity is not required. Hence, the load on the oil pump, for example, can be reduced to improve the fuel consumption.